In order to fulfil the current need for stable and specific tracers to monitor vascular- plant organic matter degradation in aquatic environments, alpha-amyrin (urs-12-en-3 beta-ol) and beta-amyrin (olean-12-en-3 beta-ol) were oxidised in vitro and their abiotic degradation products quantified in environmental samples from the Rhone River in France. Although they appear inert to photooxidation, they are clearly affected by autoxidation and the tracer potential of the resulting products was confirmed. Autoxidation of alpha- and beta-amyrins produces urs or olean-12-en-3-one, 3 beta-hydroxy-urs or olean-12-en-11- one, urs or olean-12-en-3 beta, 11 alpha-diol and urs or olean-12-en-3,11-dione. 3 beta-Hydroxy-urs-12-en-11-one and 3 beta-hydroxy- olean-12-en-11-one, the main oxidation products detected, were selected as autoxidation tracers. These compounds, specific to autoxidation, were detected in dry leaves of Smilax aspera and in suspended particulate matter samples collected in the Rhone River and evidenced the importance of autoxidation in the degradation of organic matter of terrestrial origin.

The abiotic degradation state of sea ice algae released during a late spring ice melt process was determined by sampling the underlying waters and measuring certain well-known algal lipids and their oxidation products, including those derived from epi-brassicasterol, 24-methylenecholesterol, palmitoleic acid and the phytyl side-chain of chlorophyll. More specifically, parent lipids and some of their oxidation products were quantified in suspended (collected by filtration) and sinking (collected with sediment traps at 5 and 30 m) particles from Resolute Passage (Canada) during a period of spring ice melt in 2012 and the outcomes compared with those obtained from related sea ice samples analyzed previously. Our data show that suspended cells in the near surface waters appeared to be only very weakly affected by photooxidative processes, likely indicative of a community of unaggregated living cells with high seeding potential for further growth. In contrast, we attribute the strong photooxidation state of the organic matter in the sediment traps deployed at 5 m to the presence of senescent and somewhat aggregated sea ice algae that descended only relatively slowly within the euphotic zone, and was thus susceptible to photochemical degradation. On the other hand, the increased abiotic preservation of the sinking material collected in the sediment traps deployed at 30 m, likely reflected more highly aggregated senescent sea ice algae that settled sufficiently rapidly out of the euphotic zone to avoid significant photooxidation. This better-preserved sinking material in the deeper sediment traps may therefore contribute more strongly to the underlying sediments. A three-component conceptual scheme summarizing the abiotic behavior of Arctic sea ice algae in underlying waters is proposed

PARADOXICAL EFFECTS OF TEMPERATURE AND SOLAR IRRADIANCE ON THE PHOTODEGRADATION STATE OF KILLED PHYTOPLANKTON

The aim of this paper was to study the effects of temperature and irradiance on the photodegradation state of killed phytoplankton cells. For this purpose, killed cells of the diatom Chaetoceros neogracilis RCC2022 were irradiated (photosynthetically active radiation) at 36 and 446 J · s(-1) · m(-2) (for the same cumulative dose of irradiation energy) and at two temperatures (7°C and 17°C). Analyses of specific lipid tracers (fatty acids and sterols) revealed that low temperatures and irradiances increased photooxidative damages of monounsaturated lipids (i.e., palmitoleic acid, cholesterol and campesterol). The high efficiency of type II photosensitized degradation processes was attributed to: (i) the relative preservation of the sensitizer (chlorophyll) at low irradiances allowing a longer production of singlet oxygen and (ii) the slow diffusion rate of singlet oxygen through membranes at low temperatures inducing more damages. Conversely, high temperatures and irradiances induced (i) a rapid degradation of the photosensitizer and a loss of singlet oxygen by diffusion outside the membranes (limiting type II photosensitized oxidation), and (ii) intense autoxidation processes degrading unsaturated cell lipids and oxidation products used as photodegradation tracers. Our results may explain the paradoxical relationship observed in situ between latitude and photodegradation state of phytoplankton cells

The effects of singlet oxygen ((1)O2) transfer to bacteria attached on phytodetritus were investigated under laboratory-controlled conditions. For this purpose, a nonaxenic culture of Emiliania huxleyi in late stationary phase was studied for bacterial viability. Our results indicated that only 9 ± 3 % of attached bacteria were alive compared to 46 ± 23 % for free bacteria in the E. huxleyi culture. Apparently, under conditions of low irradiance (36 W m(-2)), during the culture, the cumulative dose received (22,000 kJ m(-2)) was sufficiently important to induce an efficient (1)O2 transfer to attached bacteria during the senescence of E. huxleyi cells. At this stage, attached bacteria appeared to be dominated by pigmented bacteria (Maribacter, Roseobacter, Roseovarius), which should resist to (1)O2 stress probably due to their high contents of carotenoids. After subsequent irradiation of the culture until fully photodegradation of chlorophyll, DGGE analyses showed that the diversity of bacteria attached to E. huxleyi cells is modified by light. Photooxidative alterations of bacteria were confirmed by the increasing amounts of cis-vaccenic photoproducts (bacterial marker) per bacteria observed during irradiation time. Interestingly, preliminary chemotaxis experiments showed that Shewanella oneidensis considered here as a model of motile bacteria was attracted by phytodetritus producing or not (1)O2. This lack of repulsive effects could explain the high mortality rate of bacteria measured on E. huxleyi cells.